A polyacetal has excellent mechanical strength, slidability, heat resistance, chemical resistance, moldability (formability) and electrical characteristics, and hence is used as an engineering plastic in many fields. Further, a variety of performances have been practically required of such polyacetal accompanied with expanding applications.
On the other hand, the polyacetal has a high crystallinity and hence remarkably poor in affinity or compatibility for other materials. Therefore, it is difficult to modify or reform the polyacetal by means of blending and compounding with other polymers, additives or reinforcing agents and the like, and adhesive properties of the polyacetal with respect to a paint, a printing ink, a coloring agent or an adhesive can hardly be improved. By way of illustration, when an elastomer is blended with the polyacetal for improving impact resistance, satisfactory impact characteristics can not be imparted to the resultant molded article because of poor affinity and compatibility of the polyacetal for the elastomer. Further, the surface of the molded article is peeled off due to poor dispersibility of the elastomer, and hence the external appearance of the molded article is significantly sacrificed. Moreover, addition of a stabilizer, lubricant or other additive to the polyacetal for modification or reforming results in leaching of the additive to the surface of the molded article, and hence the appearance of the molded article is markedly deteriorated and functions of the additive can not be utilized effectively. Accordingly, application or use of the polyacetal may practically be limited to a great extent.
For the purpose of improving affinity of the polyacetal, it is useful to modify the polyacetal by introducing a modifying group into the polyacetal. As such modifying method of the polyacetal by introduction of a modifying group, there have been proposed a variety of technologies. By way of an example, Japanese Patent Publication No. 23467/1968 (JP-B-43-23467) proposes a process which comprises introducing an amino group into a base polyacetal with the use of aminoaldehyde as a comonomer. According to this process, however, boron trifluoride or other Lewis acid used as a polymerization catalyst is easily or readily reacted with the amino group, so that polymerization efficiency and hence characteristics as an engineering plastic are sacrificed.
Japanese Patent Publication No. 19425/1972 (JP-B-47-19425) proposes a process which comprises preparing a polymer with the use of a cyclic ether or cyclic acetal each having a nitro group as a comonomer, and reduction the resultant polymer to convert the nitro group into an amino group. However, in this process, the polymer is liable to be decomposed in the reducing step, and hence the characteristics of the obtained polymer are not satisfactorily high as an engineering plastic.
Japanese Patent Application Laid-open No. 21618/1991 (JP-A-3-21618) and Japanese Patent Application Laid-open No. 21619/1991 (JP-A-3-21619) propose production of a polyacetal copolymer having a hydroxyl group, an acyloxy group, for instance, with the use of a comonomer. Japanese Patent Application Laid-open No. 25238/1993 (JP-A-5-25238) offers a process which comprises copolymerizing trioxane and a cyclic ether or cyclic formal each having a nitrile group, a carboxyl group, an ester group or an amide group. According to these technologies, however, polymerization reactivity is poor so that a copolymer having a high molecular weight can hardly be obtained in a high yield. Japanese Patent Application Laid-open No. 93822/1991 (JP-A-3-93822) proposes a polyacetal copolymer having a nitrile group. However, this process requires an extremely large quantity of a polymerization catalyst and hence is not suitable for practical use.
As described above, in the processes in which copolymerization is conducted using a comonomer having a modifying group, the comonomer adversely affects on the polymerization reaction and, thereby, a smooth proceeding of the copolymerization is deteriorated and the polymerization degree can hardly be enhanced. In other words, the technologies, which comprise introducing a modifying group directly into a polyacetal with the use of a comonomer having a modifying group, each provide a decreased polymerization reactivity and a copolymer having a high molecular weight can hardly be provided in a high yield.
There has been also proposed incorporation of a polymerizable compound into a reinforced polyacetal resin composition for the purpose of improving reinforcing effect by means of a reinforcing agent. By way of an example, Japanese Patent Publication No. 6969/1985 (JP-B-60-6969) discloses a production method of a reinforced polyoxymethylene resin composition which comprises melting and kneading concurrently, in substance, a mixture prepared by adding, to 100 parts by weight of a polyoxymethylene resin, 5 to 150 parts by weight of an inorganic filler, and 0.1 to 20 parts by weight of an unsaturated polyester. This literature also describes that a radical polymerization initiator may preferably be molten and kneaded concurrently with the above mixture. Japanese Patent Publication No. 18383/1983 (JP-B-58-18383) discloses a production method of a carbon fiber-reinforced polyacetal resin composition which comprises kneading, in substantial concurrence, a mixture comprising 5 to 150 parts by weight of a carbon fiber, 0.1 to 20 parts by weight of an unsaturated polyester, 0 to 5 parts by weight of a radical polymerization initiator and 100 parts by weight of a polyacetal resin.
However, since the composition previously comprises the specific reinforcing agent, it is necessary to prepare the resin composition for each of applications. By way of an example, the carbon fiber-reinforced polyacetal resin composition shows gray through black color depending on the content of the carbon fiber so that it can not be applied to a chromatic colored molded article. Further, the molding conditions must be strictly controlled since the unsaturated polyester is apt to be cross-linked in the melting and kneading step, and appearance and quality of the molded article are deteriorated due to a granule, typically speaking, formed by crosslinking of the unsaturated polyester.
Japanese Patent Publication No. 18384/1983 (JP-B-58-18384) discloses a carbon fiber-reinforced polyacetal resin composition as produced by heating and kneading, concurrently in substance, 100 parts by weight of a polyacetal resin, 5 to 150 parts by weight of a carbon fiber, 0.01 to 5 parts by weight of a radical polymerization initiator and 0.1 to 20 parts by weight of acrylamide, N-methylolacrylamide or divinylbenzene. When the crosslinkable divinylbenzene is used in this process, disadvantages similar to the composition as produced using the unsaturated polyester would be present.
Furthermore, Japanese Patent Publication No. 36644/1984 (JP-B-59-36644) discloses a reinforced polyoxymethylene resin composition comprising a heated and kneaded product containing 100 parts by weight of polyoxymethylene, 5 to 150 parts by weight of a non-carbonaceous inorganic filler, 0.01 to 5 parts by weight of a radical polymerization initiator and 0.1 to 20 parts by weight of acrylamide or N-methylolacrylamide. Japanese Patent Application Laid-open No. 219252/1985 (JP-A-60-219252) discloses a glass fiber-reinforced polyacetal resin composition as produced by adding a glass fiber, a peroxide and a silane-based coupling agent to a polyacetal resin.
Since the unmodified polyacetal is poor in wettability with respect to a reinforcing agent or filler, the above mentioned composition can hardly provide a homogeneous dispersion of the reinforcing agent or filler with a high efficiency even when kneaded. Further, coexistence of the reinforcing agent or filler significantly sacrifices modification efficiency. Therefore, even when the composition is molded, functions or properties of the reinforcing agent or filler can not effectively be exhibited and the molded article may occasionally be imparted with excellent mechanical characteristics, electrical characteristics or other characteristics.
In particular, these technologies, in which the filler or radical polymerization initiator is employed, can hardly be referred to as effective modifying processes, since introduction efficiency of the modifying group is extremely poor and the polyacetal is decomposed or degraded.
Moreover, when the radical polymerization initiator and the polyacetal are molten and kneaded, the polyacetal resin is decomposed and degraded, due to the radical polymerization initiator, to give a decreased molecular weight. For instance, the above-mentioned Japanese Patent Application Laid-open No. 219252/1985 (JP-A-60-219252) discloses that melting and kneading of the peroxide and the polyacetal results in oxidative decomposition of the polyacetal resin due to the radical polymerization initiator. Further, Japanese Patent Application Laid-open No. 74790/1974 (JP-A-49-74790) discloses a process for controlling an average molecular weight of a polyacetal resin which comprises decomposing a polyoxymethylene by contacting the same with an organic peroxide in a molten state of the polyoxymethylene, and British Patent No. 1172741 discloses a process for reducing an average molecular weight which comprises treating a polyacetal with a peroxide in the presence of a dispersing agent or a solvent. As described above, the use of a radical polymerization initiator, in particular an organic peroxide, results in decomposition of the polyacetal, so that it is difficult to modify or reform the polyacetal by means of the radical polymerization initiator with retaining its high molecular weight, mechanical characteristics and electrical characteristics. Probably for this reason, Japanese Patent Application Laid-open No. 214212/1993 (JP-A-5-214212) proposes reforming (modification) of a polyoxymethylene resin by adding a modified polyolefin modified with a polymerizable monomer having a glycidyl group and an amide bond, without direct modification of the polyacetal.
Still more, N-methylolacrylamide may easily be self-condensed to form a dimer and be liable to be crosslinked, so that moldability or formability of the composition is deteriorated or appearance and quality of the molded article are sacrificed due to crosslinking. Further, since acrylamide is reacted with formaldehyde formed in kneading with the polyacetal to give the above N-methylolacrylamide, similar problems to the above would arise.
It is, therefore, an object of the present invention to provide a modified polyacetal which is imparted with high affinities for a polymer, reinforcing agent and/or filler and a method of producing the same.
It is another object of the invention to provide a modified polyacetal having a high molecular weight despite being introduced with a modifying group, and a method of producing such modified polyacetal.
A further object of the present invention is to provide a modified polyacetal with inhibiting a molding processabilities and recycling properties (reutility), and its production method.
It is a yet another object of the invention to provide a modified polyacetal which is introduced with a comparatively large quantity of a modifying group with a high efficiency into a base polyacetal, and a method of producing the modified polyacetal.
A still further object of the invention is to provide a production method of a modified polyacetal which insures inhibition of deterioration of its performance due to decomposition and/or degradation without interfering with formation reaction of a polyacetal, and provides an efficient modification (reforming) of the polyacetal with retaining inherently excellent performances of such polyacetal.
It is a further object of the present invention to provide a method of producing a modified polyacetal which insures, despite using a radical initiator, modification of a polyacetal with a high efficiency by the compound having a modifying group, to inhibit decrease of its molecular weight.
A yet further object of the invention is to provide a production method of a modified polyacetal which is able to provide a polymer having a molecular weight within a comparatively broad range.
It is a still another object of the invention to provide a method of producing a modified polyacetal which insures direct introduction of a modifying group into a polyacetal with a high efficiency in a simple and easy manner without the coexistence of a reinforcing agent or filler.